1. Field of the Invention
The present invention relates to thermoelectric element sheets comprising thermoelectric elements, which are preferably used in the process of semiconducting thermoelectric conversion, thermoelectric cooling in accordance with the Peltier effect, or the like.
2. Description of the Prior Art
Heretofore, thermoelectric energy conversion systems which depend on the Seebeck, Peltier or Thomson effect for conversion either way between thermal energy and electric energy have been used in the developmental fields of space, oceanography, and the like.
A thermoelectric element is one of the well-known components used in the above systems and is composed of a plurality of structural units, in which each structural unit includes a p-type thermoelectric semiconductor, an n-type thermoelectric semiconductor and an electrode connecting these semiconductors. In this case, each semiconductor has its own predetermined thickness, referred to as the thermal-flow length, which is defined as the distance between the side of the semiconductor facing a warm thermal source (i.e., a gas, liquid, solid or the like which has a relatively high temperature) and the side facing a cold thermal source (i.e., a gas, liquid, solid or the like which has a relatively low temperature).
The thermoelectric elements can be grouped into two different types, i.e., the thermoelectric generating type (e.g., a thermoelectric generator unit) and the Peltier effect type (e.g., a thermoelectric refrigerator unit). The former is responsible for converting heat into electric energy, while the latter is responsible for using electric energy to cool a material down. In spite of the different types, however, they depend on the same physical principle even though each type of thermoelectric elements works at a temperature range different from that of the other.
The conventional thermoelectric element is formed by a fabrication process including the steps of:
(i) forming an ingot of p-type (or n-type) thermoelectric semiconducting ceramic into a p-type (or n-type) thermoelectric semiconductor having a predetermined thermal-flow length by using a well-known forming means such as pressing, sintering or the like; PA1 (ii) connecting an end of the p-type thermoelectric semiconductor and an end of the n-type thermoelectric semiconductor by an electrode to make a structural unit of the thermoelectric element; and PA1 (iii) connecting a plurality of the structural units by bridging an electrode between them to make a thermoelectric element, i.e., the p-type thermoelectric semiconductor of one structural unit is connected with the n-type thermoelectric element of the adjacent structural unit. PA1 the first insulated film having a first surface on which the electrodes for connecting the thermoelectric semiconductors are arranged; and PA1 the second insulating film having a second surface on which the electrodes for connecting the structural units and the thermoelectric semiconductors are arranged, the first and second surfaces facing each other in the layered structure. PA1 where "A" is the distance between the thermoelectric semiconductors that are paired in a structural unit; PA1 "B" is the distance between the adjacent structural units; and PA1 "C" is the length of the electrode for connecting the thermoelectric semiconductors.
For the energy generation that is required in the field of space development, the thermoelectric element used in the thermoelectric generator system should be subjected to a temperature difference of 700.degree. C. between the warm thermal source and the cold thermal source. For that reason, the thermoelectric semiconductor should have a thermal-flow length of at least about 30 mm. When the thermoelectric element is fabricated by using such thick thermoelectric semiconductors, large gapped portions are formed between the semiconductors. In this case, a large amount of heat loss can occur in the conventional thermoelectric element as a result of heat radiation or heat convection from the gapped portions. To avoid such heat loss, in general, these gapped portions are filled up with a filler or the like. In spite of taking steps to deal with such disadvantages, however, the filler transmits heat and thus a large amount of heat loss cannot be avoided. Therefore, the conventional thermoelectric element has not been improved so as to decrease the amount of the heat loss from the gapped portion.
In the case of ocean thermoelectric generation using a temperature difference in seawater, the generation system should be applied as a large scaled system in spite of using a low temperature difference (i.e., about 10.degree. C.) between the warm thermal source and the cold thermal source. Therefore, there is no need to use thermoelectric semiconductors having a large thermal-flow length and thus the thermoelectric element can be easily manufactured by using thin thermoelectric semiconductors at a low cost. The width and thickness of thermoelectric semiconductors used in the field of the ocean development are relatively small compared with the semiconductors used in the field of space development, but the thermoelectric elements formed by using such narrow semiconductors also cause heat loss by heat radiation from the gapped portions formed therein.
Furthermore, it would be desirable to improve several of the properties of a conventional thermoelectric element, such as lowering the value of the heat resistance thereof. One of the ways for lowering the heat resistance is to improve the contact between a heat conductive material and the thermoelectric semiconductor. However, the thermoelectric semiconductors used in a conventional thermoelectric element are made of a very fragile ceramic material, and it is difficult to form smooth surfaces on them to make equal contact with the surface of the heat conductive material. Therefore, the heat resistance is increased. For that reason it is also difficult to avoid deformation of the thermoelectric semiconductors, which is due to thermal stress, gravity or the like, when they are employed in the thermoelectric element. Due to the above deformation, furthermore, the above contact becomes still worse. Since the thermal elements are made of a very fragile ceramic material as described above, the thermoelectric semiconductors can be easily broken or split into fragments.
Accordingly, it would be desirable to develop a novel thermoelectric element having a high reliability without causing heat loss due to heat convection, heat conduction or heat radiation in the gapped portions thereof, and it would also be desirable to manufacture the novel thermoelectric element at a low cost.